Electrophoretic display devices are a relatively new technique of pixilated display devices in which charged pigment particles are moved to generate a required pigmentation of a pixel. Examples of electrophoretic display devices may be found in the international patent application US2006/0227196. This patent application discloses a display device which comprises two substrates. One of the substrates is transparent and is provided with a single electrode which is referred to as the counter electrode. This counter electrode is in other literature also referred to as common electrode or backplane electrode. The other substrate is provided with picture electrodes which comprise row and column electrodes. A display element or pixel is associated with an intersection of a row electrode and a column electrode. Electrophoretic ink is provided between the pixel electrode and the common electrode. The electrophoretic ink comprises multiple microcapsules of about 10 to 50 microns. Each microcapsule comprises positively charged white particles and negative charge black particles suspended in a fluid. When a positive voltage is applied to the pixel electrode with respect to the common electrode, the positively charged white particles move to the side of the micro capsule directed to the transparent substrate on which the common electrode is present and a viewer will see a white display element. Simultaneously, the black particles move to the pixel electrode at the opposite side of the microcapsule where they are hidden to the viewer. By applying a negative voltage to the pixel electrode with respect to the common electrode, the black particles move to the common electrode at the side of the micro capsule directed to the transparent substrate and the display element appears dark to a viewer. When the voltage is removed, the display device remains in the acquired state and thus exhibits a bi-stable character. The electrophoretic ink display with its black and white particles is particularly useful as an electronic book. Grey scales may be created in the display device by controlling the amount of particles that move to the common electrode at the top of the microcapsules.
A known type of electrophoretic display devices comprise in-plane switching of the charged particles. Such an electrophoretic display device is, for example, known from US 2005275933. In this patent application a display panel is disclosed in which electrophoretic material is sandwiched between a first and a second substrate. A pixel of said display further comprises a first and second electrode for locally controlling the material of said electrophoretic layer. The first and second electrodes are positioned on essentially the same distance from the first substrate so that an essentially lateral field is generated in said electrophoretic layer which is used to move the particles in and out of the light path of the pixel of the display, as such defining the transmission of the pixel.
A benefit of the in-plane switching of charged particles is that the electrophoretic display device may comprise a transparent state, allowing a choice of reflector or possibly backlight. However, it is relatively difficult to control the electrical field and particle motion distribution accurately enough to provide a homogenous pixel absorbance in the “dark” state.